Sodium bicarbonate (NaHCO3) is an important alkali product with a wide range of applications including human food, animal feed, flue gas treatment, and chemical industries. The production of sodium bicarbonate is currently almost entirely made by the carbonation of solid or aqueous solutions of sodium carbonate with gaseous CO2 either produced in situ in a soda ash plants or purchased independently.
Sources of sodium carbonate (Na2CO3) for making sodium bicarbonate may be produced by the Solvay ammonia synthetic process, the ammonium chloride process, and the trona-based processes. Sodium carbonate, also known as soda ash, is one of the largest volume alkali commodities made world wide with a total production in 2008 of 48 million tons. Sodium carbonate finds major use in the glass, chemicals, detergents industries, and also in the sodium bicarbonate production industry.
About 90% of the total U.S. soda ash production ash is produced from trona ore deposits in the Green River Basin in Wyoming. Trona ore is a mineral that contains up to 99% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). Trona-based soda ash is obtained from trona ore deposits either by conventional underground mining techniques, by solution mining or lake waters processing. A typical analysis of the trona ore in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.4NaHCO334.4H2O (crystalline and free moisture)15.4NaCl0.01Na2SO40.01Fe2O30.14Insolubles6.3Organics0.3
The ‘monohydrate’ commercial process is frequently used to produce soda ash from trona. In the production of soda ash, crushed trona ore is calcined (e.g., heated) to decompose the sodium sesquicarbonate to sodium carbonate.2Na2CO3.NaHCO3.2H2O→3Na2CO3+5H2O(g)+CO2.(g)
The calcination drives off water of crystallization and forms crude soda ash. The calcined ore is dissolved in water or dilute sodium carbonate liquor to give a saturated solution of ˜30% Na2CO3 (depending upon the temperature of the solution) containing insoluble material and impurities originating from trona. The insoluble material is separated from the resulting saturated solution.
According to a preferred soda ash production process termed ‘monohydrate process’, this clear sodium carbonate-containing solution can be fed to a monohydrate evaporative crystallizer. As this solution is heated, evaporation of water takes place effecting the crystallization of sodium carbonate into sodium carbonate monohydrate crystals (Na2CO3.H2O). The monohydrate crystals are removed from the mother liquor and then dried to convert it to anhydrous soda ash (Na2CO3). The mother liquor is recycled back through a crystallizer circuit for further processing into sodium carbonate monohydrate crystals. To avoid contamination and deterioration of crystal shape and hardness by impurities and to prevent the buildup of these impurities in the crystallizer, a portion of the crystallizer liquor must be purged. This can result in a loss of up to about 10% of the soda values. The purge liquor includes sodium carbonate as well as impurities, such as organics, sodium bicarbonate, sodium chloride, sodium sulfate, and sodium silicate. This purge liquor typically contains ca. 23-28% sodium carbonate, 1-4% sodium bicarbonate and minor amounts of impurities such as organics, sodium chloride, sodium sulfate, and silicates.
In the production of sodium bicarbonate, the CO2 feedstock can have different origins. The CO2 feed for the sodium bicarbonate production may come from at least one of the following CO2 sources selected from the group consisting of:                1/ from a natural gas plant, after having been concentrated and being carried through a pipeline to the sodium bicarbonate production;        2/ from a gas effluent comprising diluted CO2 which originates from a soda ash plant, for instance from a calciner system used to calcine mechanically-mined trona ore; and        3/ from a vessel (e.g., tank, railcar) comprising purified liquid CO2 (at least 99.5% CO2).        
For option 1/, the operators of the natural gas plant must remove CO2 from the natural gas which is then liquefied for transportation via natural gas pipeline. This CO2 source can be used by a sodium bicarbonate producer if the natural gas plant is proximate (for example within 10 miles) to the sodium bicarbonate production but the recovered CO2 must be concentrated (for example through an amine process), and delivering this CO2 source to the sodium bicarbonate production plant typically requires a liquid CO2 pipeline to be constructed. The further apart the natural gas plant and the sodium bicarbonate production plant are from each other, the more expensive in capital expenditures this option becomes. This option also suffers from unplanned/planned downtimes at the natural gas plant when the production of enriched liquid CO2 is stopped.
For option 2/, the soda ash plant may provide a diluted CO2 gas effluent for example from a soda ash calciner system including its boiler stack and/or from a debicarbonation unit in which the content in sodium bicarbonate of a sodium carbonate-containing stream is reduced. This source of CO2 generally requires CO2 enrichment. The enrichment may be carried out by an amine process which involves scrubbing of CO2 from the gas effluent with an amine solvent and steam regeneration of the CO2-loaded amine solvent to recover CO2.
For option 3/, this is by far the most convenient option for highly-pure CO2 source because the sodium bicarbonate production operators do not need to invest in CO2 concentration equipment, but it is also the most expensive option and requires continual delivery by liquid-CO2 trucks or railcars to the plant.
Because the cost of raw material CO2 plays an important part in the economic picture for the entire sodium bicarbonate production plant, there is still a need to further reduce the cost of production for crystalline sodium bicarbonate, without impairing operation conditions of the process.
Sodium sulfite has a variety of commercial uses such as a disinfectant or bleaching (decoloring) agent for fabrics and paper. It is also used as a preservative in food.
Sodium sulfite (Na2SO3) can be manufactured in a number of ways. Commonly, sodium sulfite is crystallized from a solution of sodium sulfite. Sodium sulfite can be prepared by reacting sulfur dioxide (SO2) in an aqueous, alkaline solution, such as solutions of sodium hydroxide, also known as caustic soda (NaOH), or sodium carbonate (Na2CO3). Sodium sulfite can react with sulfur dioxide to produce sodium bisulfite. When sodium carbonate is used as a reactant, carbonic acid (H2CO3) is a by-product of the reaction. The carbonic acid evolves as carbon dioxide (CO2). The sodium sulfite liquor formed by reaction is then fed to a crystallization system, where the sodium sulfite is crystallized. The crystallization system includes a sulfite evaporative crystallizer and a crystallizer heater in a heater circulation loop connected to the sulfite evaporative crystallizer. The sulfite liquor is generally moved by the help of a circulating pump in the heater circulation loop. These sulfite crystals are removed from the crystallizer and dried in a rotary dryer. The CO2 byproduct is generally vented out of the sodium sulfite production process and contributes to greenhouse gas emissions of this plant.
A variety of processes have been disclosed for producing sodium sulfite and address the removal of CO2 byproduct. U.S. Pat. No. 2,245,697 entitled “Manufacture of alkali metal sulfites” discloses a process for making sodium sulfite and teaches that water vapor and air introduced into the reactor as diluent in the SO2 feed is vented out along with CO2 that is present in appreciable amounts under certain acidic reactor conditions. The patent also teaches that the liquor is “gassed” with SO2 feed so that all of the CO2 is “expelled”. U.S. Pat. No. 2,080,528 entitled “Process of manufacturing anhydrous sodium sulfite” discloses reacting sodium carbonate with sulfur dioxide to make a sodium sulfite liquor that is then boiled to remove residual CO2 gas. U.S. Pat. No. 2,719,075 entitled “Purification of alkali metal sulfite liquors” discloses introducing air into a sodium bisulfite liquor to remove CO2. U.S. Pat. No. 1,937,944 entitled “Manufacture of sulphites” discloses a process for manufacturing sodium sulfite from sodium carbonate and sulfur dioxide and teaches that the reactor liquor is circulated in the absorbing tower until all the CO2 has “passed off”. FR 2534571 entitled “Process and device for production of sodium and potassium sulfite” teaches the use of heat transfer equipment in the consecutive preparation of Na2SO3, NaHSO3, and Na2S2O5.
When CO2 formed in an alkali production process, such as in the sodium sulfite process, is basically a waste stream which contributes to greenhouse gas emissions, it would be advantageous to jointly manufacture such alkali product and crystalline sodium bicarbonate, in which the CO2, liberated as byproduct in gaseous form from the alkali production process, is used as a CO2 source in the production of sodium bicarbonate from sodium carbonate.